Afocal optical system

Synonyms :
Telescopic system

An afocal system is formed by the combination of two focal systems. The rear focal point of the first system is coincident with the front focal point of the second system. Rays parallel to the axis in object space are conjugate to rays parallel to the axis in image space.


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Air interface

The air gap between the data carrier and read-write head through which data and energy are transmitted. is based on orthogonal frequency division multiplexing (OFDM).        

Airy rings

represent a diffraction pattern from a circular aperture illuminated by a coherent beam of light. The central white spot is called the Airy disk, its radius corresponds to the radius of the first dark ring. A light beam incident on a small circular aperture becomes diffracted and its light intensity pattern after passing through the aperture is a diffraction pattern with circular bright rings (called Airy rings). If the screen is far away from the aperture, this would be  a Fraunhofer diffraction pattern.


is the central bright circular region of the pattern produced by light diffracted when passing through a small circular aperture. The central disk is surrounded by less intense concentric rings, so light intensity takes local maxima and mimina while it decreases away from the center. The size of an Airy disk depends on the WaveLength of the light and the size of the aperture.



Also Known As:





244 H / c. 858 CE

Harran, Bilad al-Sham '' Syrian ''

317 H / 929 CE (71)
Qasr al-Jiss, near Samarra '' Iraq '' 


Scientist and Inventor

Al-Battani ''Abu Abdallah Mohammad ibn Jabir ibn Sinan al-Raqqi al-Harrani al-Sabi al-Battani '' is one of the famous Muslim scholars of mathematics and planetary studies. He was born in the age known as the Golden Age of Islam. With his deep interest in trigonometry – the branch of math that deals with three sides and three angles, he found relations between trigonometric signs like sine, cosine, and tangent. This led him to determine the changing position of the Sun, Moon, night and, day in a year. He was a brilliant observer of solar and lunar eclipses – events when either Moon or Earth receive a shadow. Al-Battani spent around forty years in Raqqa, a city of Central Syria, where he made vast contributions to science. He also wrote a book on planetary tables known as Kitab Al-Zig. This book was later on used by many known scholars like Nasir Al-Din Tusi, John Kepler, and Nicolaus Copernicus for research on astronomy and trigonometry. He paved the way for other scholars to work in the future to discover unknown concepts like orbits in space. Al-Battani was the first to determine that the solar year is equal to 365 days with an additional 5 hours and 45 minutes. This figure of the solar year is in accordance with modern science’s calculation of the solar year. He was first to know and explain the concept of Equinoxes. Equinoxes are times when the Sun is over the line at the middle point of Earth when the length of day and night becomes equal. His works on the Sun, Moon, and their yearly changing positions made him arrange the Islamic Lunar Calendar – a calendar based on the monthly changing position of the Moon. 

Al-Battani’s expertise on trigonometry made him earn the title of ‘Ptolemy of the Arabs’.
His book Kitab Al-Zig extended and even corrected most of the planetary calculations made by Ptolemy. Moreover, he added tables of planets in his book that John Kepler used in his works on planetary motions a few centuries later.
Al-Battani determined the relation between trigonometric terms and expressed them in equations like secant, cosecant, tangent, and cotangent.